DISEASE CHARACTERISTICS: Histologically confirmed asymptomatic, progressive, metastatic adenocarcinoma of the prostate Progressive disease defined as increasing prostate-specific antigen (PSA) levels from 2 measurements at least 2 weeks apart PSA greater than 5 ng/mL Continued medical means of gonadal ablation (e.g., luteinizing hormone releasing hormone (LHRH)) required No known CNS metastases unless previously treated by surgery or radiotherapy and stable, asymptomatic, and not requiring steroids and anticonvulsants